During recovery of oil from oil reservoirs, typically only minor portions of the original oil in the oil-bearing subterranean strata is recovered by primary recovery methods which use only the natural forces present in the oil reservoir. Secondary oil recovery methods such as water flooding, that is, the injection of water through injection wells into the oil reservoir, have been used to force oil through the subterranean strata toward production wells and thus improve oil recovery (Hyne, N. J., 2001, “Non-technical guide to petroleum geology, exploration, drilling, and production”, 2nd edition, Pen Well Corp., Tulsa, Okla., USA). One problem commonly encountered with water flooding operations is that the heterogeneity of the subterranean strata can lead to reduced sweep efficiency during water flooding. In other words, water preferentially channels through the more porous strata of the oil reservoir as it travels from the injection well to the production wells and creates watered-out strata, and bypasses other subterranean oil-bearing subterranean strata that are not watered-out, thereby reducing the efficiency of oil recovery from the strata that is not watered-out.
It is known to use Microbial Enhanced Oil Recovery (MEOR) to assist oil recovery (Brown, L. R., Vadie, A. A., Stephen, O. J. SPE 59306, SPE/DOE Improved Oil Recovery Symposium, Oklahoma, 3-5—Apr., 2000). Viable microorganisms can be injected into an oil reservoir where they can adhere to the surfaces of pores and channels in the rock or sand matrices in the watered-out zones and plug them to prevent water channeling towards watered-out zones during water flooding, which targets the water flow towards a larger area of the oil-bearing subterranean strata that has not been watered-out (Ramkrishna, S., Prog. Energy Com. Sci., 34: 714-724, 2008).
One of the problems associated with MEOR is premature activity of exogenously introduced microorganisms, or native microorganisms, as soon as nutrient solutions are added to an injection well which results in premature bioplug formation and blocking of the pores close to the injection well bore. In this situation components introduced into the injection well cannot move into the oil-bearing subterranean strata to promote MEOR.
WO2009017810A1 describes a method of enhanced oil recovery from an oil-bearing rock formation. The method involves introducing a consortium of microorganisms and a nutrient solution into an injection well. The consortium is maintained for an amount of time sufficient for the growth and colonization of the microbes in the reservoir to displace oil.
The commonly owned and co-pending U.S. Patent Application Publication US 20100081585A1, which is herein incorporated by reference in its entirety, describes controlling activation of microorganisms in MEOR and bioremediation processes and relies on formulations that are inhibitory, at applied concentrations, to both native and introduced microbes when applied to the site. These formulations prevent microbial growth and activation until the inhibitory agent has dissipated.
U.S. Pat. No. 4,947,923 discloses a process for MEOR comprising injecting bacteria into a well bore and injecting a nutrient source to cause bacteria to grow and selectively plug the formation. The nutrient used is capable of flowing downhole and providing a nutrient source of phosphate, e.g., tripolyphosphate, without precipitation on contact with connate water.
For successful MEOR and bioremediation operations, prevention of premature bioplug formation and blocking of the pores close to the injection well bore remains a problem to be solved.